In the field of vibration testing, electromagnetic actuators, also called shakers, are typically used in the production environment to test items at varying levels of force, velocity and displacement and over varying periods of time.
Some shakers are constructed to apply low levels of force over relatively short periods of time while others are made for more extreme conditions, such as are necessary for shock testing. To be suitable to test items under heavy loads and at very high stress levels over long continuous periods, shakers must be extremely robust and highly reliable.
Examples of known air-cooled electromagnetic actuators are the applicant's existing V830, V850, V875 and V895 models. FIG. 1 illustrates a general construction of such an actuator. The armature 1 is adapted to vibrate relative to the body or stator 2 and is suspended from the stator by suspension members 7. The armature 1 includes armature coil 4 which is located in an annular air gap 3. Two electromagnets, 3a running in opposition are provided which generate D.C. magnetic fields across the air gap to supply the motive force. The coil 4 is energised by an alternating current so that it moves relative to the stator 2, causing the armature 1 to vibrate at the frequency of the applied alternating current. An article to be vibration tested may be placed directly on top of the armature normal to its axis of vibration, or on a work table carried by the armature. Alternatively, the article to be tested may be placed on a horizontal table coupled to the armature when horizontal vibration testing is to be carried out. An example of such horizontal vibration testing is described in U.S. Pat. No. 4,489,612.
In a shock test, a half wave pulse such as a square or sine wave is applied with a pulse width of typically 1 to 25 ms. The maximum level of shock is limited by one of three factors, depending on the pulse width. For low frequency testing, at pulse widths longer than for example 18 ms, the limit tends to be set by the maximum possible displacement of the armature assembly within the actuator. At medium frequencies of testing, the limit tends to be set by the maximum supply voltage, which depends on the performance of the power amplifier supplying the vibrator. This voltage limitation typically occurs for pulse widths of 5 to 18 ms. High frequency testing tends to be limited by the maximum current that the amplifier can supply, and this limitation typically occurs for pulse widths less than 5 ms.
High frequency performance can be improved by increasing the current output of the amplifier. Typically amplifiers have power modules which may be connected in parallel, and the current can be increased almost indefinitely by multiplying the number of power modules. However, the maximum supply voltage from such a parallel arrangement remains unchanged.
Enhancing the system performance for a specific range of voltage-limited shock tests can be achieved using dedicated armatures, with special windings. Thus the armature winding has to be selected to suit the type of body to be tested by the vibration, and to suit the frequency and amplitude of vibration.
An alternative solution to the voltage limitation that has been proposed is to use matching transformers, but these are expensive. Accordingly, the purpose of the invention is to find an economic solution to the voltage limitation.